Golf ball

ABSTRACT

Golf ball  2  has spherical core  4 , mid layer  6  situated on the external side of the core  4 , and cover  8  situated on the external side of the mid layer  6 . The mid layer  6  is constituted with a resin composition. This resin composition contains 100 parts by weight of an ionomer resin (A) and 25 parts by weight or more and 100 parts by weight or less of a fatty acid metal salt (B). This ionomer resin (A) is a neutralized product of a ternary copolymer of ethylene, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acid ester, with a metal ion. The fatty acid metal salt (B) has 18 to 30 carbon atoms. The weight Wm of the mid layer  6  is greater than the weight Wc of the cover  8 . The sum (Tm+Tc) of the thickness Tm of the mid layer  6  and the thickness Tc of the cover  8  is 1.0 mm or greater and 3.0 mm or less.

This application claims priority on Patent Application No. 2009-293742 filed in JAPAN on Dec. 25, 2009. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. More particularly, the present invention relates to multi-piece golf balls having a core, a mid layer and a cover.

2. Description of the Related Art

Top requirement for golf balls by golf players is their flight performances. The flight performances correlate with resilience performances of the golf ball. Hitting of a golf ball that is excellent in resilience performance leads to a high-speed flight, whereby a great flight distance is attained.

For attaining a great flight distance, an appropriate trajectory height is required. The trajectory height varies depending on the spin rate and launch angle. Golf balls which achieve a high trajectory due to a high spin rate are accompanied by insufficient flight distance. Golf balls which achieve a high trajectory due to a great launch angle can attain a great flight distance. By employing an outer-hard/inner-soft structure in a golf ball, a low spin rate and a great launch angle can be both achieved.

General golf balls have a core, a mid layer and a cover. The core is molded by crosslinking a rubber composition. The mid layer and the cover are constituted with a resin composition. The cross-linked rubber is highly responsible for achieving the resilience performance of the golf ball. By employing a large core, high resilience of the golf ball can be achieved.

Typically, an ionomer resin is used for the mid layer. The ionomer resin is highly elastic. Golf balls having a mid layer including an ionomer resin are excellent in resilience performances. In particular, an ionomer resin having a high degree of neutralization can be responsible for achieving the resilience performances.

Japanese Unexamined Patent Application, Publication No. H7-24085 (equivalent to U.S. Pat. No. 5,553,852) discloses a golf ball having a center, a mid layer and a cover. The specific gravity of this mid layer is less than the specific gravity of the center. In this golf ball, the weight distributes to be biased inwardly. This golf ball leads to occurrence of an excessive spin.

Japanese Unexamined Patent Application, Publication No. H11-253578 (equivalent to U.S. Pat. No. 6,129,640) discloses a golf ball having a core, a mid layer and a cover. This mid layer contains a polyurethane as a principal component. This mid layer deteriorates the resilience performance of the golf ball. This golf ball is inferior in the flight performance.

Japanese Unexamined Patent Application, Publication No. 2006-289059 (equivalent to US 2006/0211517) discloses a golf ball having a core, a mid layer, a reinforcement layer and a cover. In this golf ball, the weight distributes to be biased inwardly. This golf ball leads to occurrence of an excessive spin.

Japanese Unexamined Patent Application, Publication No. 2006-289060 (equivalent to US 2006/0211517) discloses a golf ball having a core, a mid layer, a reinforcement layer and a cover. In this golf ball, the weight distributes to be biased inwardly. This golf ball leads to occurrence of an excessive spin.

Japanese Unexamined Patent Application, Publication No. 2000-157646 (equivalent to U.S. Pat. No. 6,329,458) discloses a resin composition for golf balls containing an ionomer resin and a metal soap. Japanese Unexamined Patent Application (translation of a PCT Application), Publication No. 2002-514112 (equivalent to U.S. Pat. No. 6,100,321) discloses a resin composition for golf balls containing an ionomer resin and a stearic acid metal salt.

A golf ball in which a large core is employed has a thin mid layer. When a thin mid layer is molded, a narrow gap is provided between the cavity face of the mold, and the core. Therefore, flow of a molten resin composition may be impaired in the gap between the cavity face of the mold, and the core. A composition containing an ionomer resin having a high degree of neutralization tends to have an inferior flow property. Golf balls having a large core, and in which an ionomer resin having a high degree of neutralization is used, bare is likely to be generated on the mid layer due to unfavorable flow. This golf ball is inferior in productivity.

An object of the present invention is to provide golf balls that are excellent in various performances.

SUMMARY OF THE INVENTION

A golf ball according to one aspect of the present invention has a core, a mid layer situated on the external side of the core, and a cover situated on the external side of the mid layer. The weight Wm of the mid layer is greater than the weight Wc of the cover. The sum (Tm+Tc) of the thickness Tm of the mid layer and the thickness Tc of the cover is 1.0 mm or greater and 3.0 mm or less. The mid layer is constituted with a resin composition. This resin composition contains 100 parts by weight of an ionomer resin (A) and 25 parts by weight or more and 100 parts by weight or less of a fatty acid metal salt (B). The ionomer resin (A) is a neutralized product of a ternary copolymer of ethylene, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acid ester, with a metal ion. The fatty acid metal salt (B) has 18 to 30 carbon atoms.

In the golf ball according to the present invention, the ionomer resin (A) is neutralized with a fatty acid metal salt (B) having 18 to 30 carbon atoms. The mid layer containing this ionomer resin serves in achieving a superior resilience performance of the golf ball. The resin composition containing the ionomer resin (A) and the fatty acid metal salt (B) is excellent in a flow property. The golf ball having a mid layer in which this resin composition is used is excellent in the productivity.

Preferably, the difference (Wm−Wc) between the weight Wm and the weight Wc is 0.1 g or greater and 3 g or less.

Preferably, the difference (Gm−Gc) between the specific gravity Gm of the mid layer and the specific gravity Gc of the cover is greater than 0 and no greater than 1.15. Preferably, the specific gravity Gm of the mid layer is 1.05 or greater and 2.0 or less. Preferably, the specific gravity Gc of the cover is 0.90 or greater and 1.05 or less.

Preferably, the volume Vm of the mid layer is less than the volume Vc of the cover. Preferably, the difference (Vc−Vm) between the volume Vc and the volume Vm is no less than 0.1 cm³.

Preferably, the metal ion of the ionomer resin (A) is a magnesium ion, a zinc ion or a sodium ion, whereas the fatty acid component of the fatty acid metal salt (B) is stearic acid, behenic acid or montanic acid, and the metal ion of the fatty acid metal salt (B) is a magnesium ion, a calcium ion or a barium ion.

Preferably, the Shore D hardness Hm of the mid layer is 30 or greater and 60 or less. The resin composition of the mid layer has a melt flow rate of preferably 1.0 g/10 min or greater and 50 g/10 min or less. The resin composition of the mid layer has a modulus of flexural rigidity of preferably 100 MPa or greater and 300 MPa or less. The resin composition of the mid layer has a modulus of impact resilience of preferably no less than 40%.

Preferably, the ratio (Pn2/Pn1) of the degree of neutralization Pn2 of the resin composition of the mid layer to the degree of neutralization Pn1 of the ionomer resin (A) not mixed with the fatty acid metal salt (B) is no less than 1.2.

Preferably, the difference (Hs−Ho) between the JIS-C hardness Hs of the surface of the core and the JIS-C hardness Ho of the central point of the core is no less than 20. Preferably, the JIS-C hardness Ho of the central point of the core is 25 or greater and 70 or less. Preferably, the JIS-C hardness Hs of the surface of the core is 60 or greater and 95 or less. The core has a specific gravity of preferably 1.00 or greater and 1.20 or less.

Preferably, the thickness Tc of the cover is no greater than 2.0 mm. Preferably, the Shore D hardness of the cover is 50 or greater and 75 or less.

The core may have a center, and an envelope layer situated on the external side of the center.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partially cut off cross-sectional view illustrating a golf ball according to one embodiment of the present invention

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail according to the preferred embodiments with appropriate references to the accompanying drawing.

A golf ball 2 shown in FIG. 1 has a spherical core 4, a mid layer 6 situated on the external side of the core 4, and a cover 8 situated on the external side of the mid layer 6. A large number of dimples 10 are formed on the surface of the cover 8. Of the surface of the golf ball 2, a part other than the dimples 10 is land 12. This golf ball 2 has a paint layer and a mark layer on the external side of the cover 8 although these layers are not shown in the FIGURE. The golf ball 2 may have other layer between the core 4 and the mid layer 6. The golf ball 2 may have other layer between the mid layer 6 and the cover 8.

This golf ball 2 has a diameter of from 40 mm to 45 mm. From the standpoint of conformity to a rule defined by United States Golf Association (USGA), the diameter is preferably no less than 42.67 mm. In light of suppression of the air resistance, the diameter is preferably no greater than 44 mm, and particularly preferably no greater than 42.80 mm. The weight of this golf ball 2 is 40 g or greater and 50 g or less. In light of attainment of great inertia, the weight is preferably no less than 44 g, and particularly preferably no less than 45.00 g. From the standpoint of conformity to a rule defined by USGA, the weight is preferably no greater than 45.93 g.

Preferably, the core 4 is obtained through crosslinking of a rubber composition. Illustrative examples of preferable base rubber include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers and natural rubbers. In light of the resilience performance, polybutadienes are preferred. When other rubber is used in combination with a polybutadiene, it is preferred that the polybutadiene is included as a principal component. The percentage of the amount of the polybutadiene relative to the total amount of the base rubber is preferably no less than 50% by weight, and particularly preferably no less than 80% by weight. The percentage of cis-1,4 bonds in the polybutadiene is preferably no less than 40%, and particularly preferably no less than 80%.

The rubber composition for use in the core 4 contains a co-crosslinking agent. The co-crosslinking agent serves in achieving a high resilience of the core 4. Preferable examples of the co-crosslinking agent in light of the resilience performance include monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specific examples of the preferable co-crosslinking agent include zinc acrylate, magnesium acrylate, zinc methacrylate and magnesium methacrylate. In light of the resilience performance, zinc acrylate and zinc methacrylate are particularly preferred.

In light of the resilience performance of the golf ball 2, the amount of the co-crosslinking agent is preferably no less than 20 parts by weight, and particularly preferably no less than 25 parts by weight relative to 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the co-crosslinking agent is preferably no greater than 50 parts by weight, and particularly preferably no greater than 40 parts by weight relative to 100 parts by weight of the base rubber.

Preferably, the rubber composition for use in the core contains an organic peroxide together with the co-crosslinking agent. The organic peroxide serves as a crosslinking initiator. The organic peroxide is responsible for achieving the resilience performance of the golf ball 2. Examples of suitable organic peroxide include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.

In light of the resilience performance of the golf ball 2, the amount of the organic peroxide is preferably no less than 0.1 parts by weight, more preferably no less than 0.3 parts by weight, and particularly preferably no less than 0.5 parts by weight relative to 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic peroxide is preferably no greater than 3.0 parts by weight, more preferably no greater than 2.0 parts by weight, and particularly preferably no greater than 1.5 parts by weight relative to 100 parts by weight of the base rubber.

Preferably, the rubber composition for use in the core contains an organic sulfur compound. Illustrative examples of preferable organic sulfur compound include mono-substituted forms such as diphenyl disulfide, bis(4-chlorophenyl) disulfide, bis(3-chlorophenyl) disulfide, bis(4-bromophenyl) disulfide, bis(3-bromophenyl) disulfide, bis(4-fluorophenyl) disulfide, bis(4-iodophenyl) disulfide and bis(4-cyanophenyl) disulfide; di-substituted forms such as bis(2,5-dichlorophenyl) disulfide, bis(3,5-dichlorophenyl) disulfide, bis(2,6-dichlorophenyl) disulfide, bis(2,5-dibromophenyl) disulfide, bis(3,5-dibromophenyl) disulfide, bis(2-chloro-5-bromophenyl) disulfide and bis(2-cyano-5-bromophenyl) disulfide; tri-substituted forms such as bis(2,4,6-trichlorophenyl) disulfide and bis(2-cyano-4-chloro-6-bromophenyl) disulfide; tetra-substituted forms such as bis(2,3,5,6-tetrachlorophenyl) disulfide; and penta-substituted forms such as bis(2,3,4,5,6-pentachlorophenyl) disulfide and bis(2,3,4,5,6-pentabromophenyl) disulfide. The organic sulfur compound is responsible for achieving the resilience performance. Particularly preferred organic sulfur compounds are diphenyl disulfide, and bis(pentabromophenyl) disulfide.

In light of the resilience performance of the golf ball 2, the amount of the organic sulfur compound is preferably no less than 0.1 parts by weight, and particularly preferably no less than 0.2 parts by weight relative to 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic sulfur compound is preferably no greater than 1.5 parts by weight, more preferably no greater than 1.0 parts by weight, and particularly preferably no greater than 0.8 parts by weight relative to 100 parts by weight of the base rubber.

Into the core 4 may be blended a filler for the purpose of adjusting the specific gravity and the like. Illustrative examples of suitable filler include zinc oxide, barium sulfate, calcium carbonate and magnesium carbonate. The amount of the filler is determined ad libitum so that the intended specific gravity of the core 4 can be accomplished. Particularly preferable filler is zinc oxide. Zinc oxide serves not only to adjust the specific gravity but also as a crosslinking activator. A variety of additives such as an anti-aging agent, a coloring agent, a plasticizer, a dispersant and the like may be added to the core 4 in an adequate amount as needed. Crosslinked rubber powders or synthetic resin powders may be blended into the core 4.

In light of the durability, JIS-C hardness Ho of the central point of the core 4 is preferably no less than 25, more preferably no less than 30, and particularly preferably no less than 35. In light of suppression of the spin, the central hardness Ho is preferably no greater than 70, more preferably no greater than 65, and particularly preferably no greater than 60. The central hardness Ho is measured by pushing a JIS-C type hardness scale on a central point a section of a hemisphere which had been obtained by cutting the core 4. For the measurement, an automated rubber hardness tester (“P1”, trade name, available from Kobunshi Keiki Co., Ltd.) equipped with this hardness scale is used.

In light of the resilience performance, the surface hardness Hs of the core 4 is preferably no less than 60, more preferably no less than 70, and particularly preferably no less than 75. In light of the feel at impact, the surface hardness Hs is preferably no greater than 95, more preferably no greater than 90, and particularly preferably no greater than 85. The surface hardness Hs is measured by pushing a JIS-C type hardness scale on the surface of the core 4. For the measurement, an automated rubber hardness tester (“P1”, trade name, available from Kobunshi Keiki Co., Ltd.) equipped with this hardness scale is used.

In light of suppression of the spin, the difference (Hs−Ho) between the surface hardness Hs and the central hardness Ho is preferably no less than 10, more preferably no less than 15, and particularly preferably no less than 20. In light of the durability of the golf ball 2, the difference (Hs−Ho) is preferably no greater than 30.

The core 4 has a specific gravity of no greater than 1.20. This core 4 enables the weight distribution of the golf ball 2 to be biased outwardly. Thus distributed golf ball 2 has a great moment of inertia. The spin is suppressed according to this golf ball 2. In this respect, the specific gravity is more preferably no greater than 1.18, and particularly preferably no greater than 1.16. The specific gravity is preferably no less than 1.00.

The core 4 has a diameter of preferably no less than 36.0 mm. Due to this core 4, the mid layer 6 is located away from the central point. As described later, this mid layer 6 has a great specific gravity. A great moment of inertia is attained by the location of the mid layer 6 having a great specific gravity away from the central point. The spin is suppressed according to this golf ball 2. In this respect, this diameter is more preferably no less than 38.0 mm, and particularly preferably no less than 39.5 mm. In light of moldability of the mid layer 6 having a sufficient thickness, the diameter is preferably no greater than 41 mm.

The core 4 has a weight of preferably 32 g or greater and 39 g or less. The crosslinking temperature of the core is usually 140° C. or higher and 180° C. or lower. The crosslinking time period of the core 4 is usually 10 minutes or longer and 60 minutes or shorter. The core 4 may have a rib on the surface thereof. The core 4 may be composed of a spherical center, and an envelope layer situated on the external side of this center.

The mid layer 6 is constituted with a resin composition. A resin composition particularly suited for use in the mid layer 6 contains 100 parts by weight of a ionomer resin (A), and 25 parts by weight or more and 100 parts by weight or less of a fatty acid metal salt (B). This ionomer resin (A) is a neutralized product of a ternary copolymer of ethylene, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acid ester, with a metal ion. The aforementioned fatty acid metal salt (B) has 18 to 30 carbon atoms.

In this resin composition, the metal included in the fatty acid metal salt (B) is speculated to react with carboxyl groups included in the ternary copolymer. Moreover, in this resin composition, metal ions released from the fatty acid metal salt (B) are speculated to react with the carboxyl groups included in the ternary copolymer. The degree of neutralization of the ternary copolymer in this composition is higher than the degree of neutralization of the ionomer resin (A) that is a source material. This resin composition can be responsible for the resilience performance of the golf ball 2. The fatty acid included in the fatty acid metal salt (B) exerts a lubricating action. The resin composition containing this fatty acid metal salt (B) has a superior flow property. The golf ball 2 in which this resin composition is used is excellent in the resilience performance and productivity.

Illustrative examples of the α,β-unsaturated carboxylic acid in the ionomer resin (A) include acrylic acid, methacrylic acid, fumaric acid, maleic acid and crotonic acid. Acrylic acid and methacrylic acid are preferred. In light of the resilience performance of the golf ball and the flow property of the resin composition, the α,β-unsaturated carboxylic acid has preferably 3 or more and 8 or less carbon atoms.

Illustrative examples of the α,β-unsaturated carboxylic acid ester in the ionomer resin (A) include methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, methyl fumarate, ethyl fumarate, propyl fumarate, n-butyl fumarate, isobutyl fumarate, methyl maleate, ethyl maleate, propyl maleate, n-butyl maleate and isobutyl maleate. Acrylic acid esters and methacrylic acid esters are preferred. In light of the resilience performance of the golf ball and the flow property of the resin composition, the α,β-unsaturated carboxylic acid ester has preferably 2 or more and 22 or less carbon atoms.

In light of the resilience performance, particularly preferable ternary copolymer is an ethylene-(meth)acrylic acid-(meth)acrylic acid ester copolymer.

The content of ethylene in the ionomer resin (A) is preferably no less than 65% by weight, and particularly preferably no less than 70% by weight. This content is preferably no greater than 90% by weight, and particularly preferably no greater than 85% by weight.

The content of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the ionomer resin (A) is preferably no less than 2% by weight, and particularly preferably no less than 3% by weight. This content is preferably no greater than 30% by weight, and particularly preferably no greater than 25% by weight.

The content of the α,β-unsaturated carboxylic acid ester in the ionomer resin (A) is preferably no less than 1% by weight, and particularly preferably no less than 3% by weight. This content is preferably no greater than 25% by weight, and particularly preferably no greater than 20% by weight.

The degree of neutralization of the ionomer resin (A) not mixed with the fatty acid metal salt (B) is preferably 20% by mole or greater and 90% by mole or less. Due to the ionomer resin (A) having a degree of neutralization of no less than 20% by mole, a superior resilience performance golf ball 2 can be achieved. In this respect, the degree of neutralization is particularly preferably no less than 30% by mole. The ionomer resin (A) having a degree of neutralization of no greater than 90% by mole is excellent in the flow property. In this respect, the degree of neutralization is particularly preferably no greater than 85% by mole. The degree of neutralization Pn1 is determined by the following mathematical expression.

Pn1=(Mn/M)·100

In this mathematical expression, M represents the number of moles of the carboxyl groups included in the ionomer resin (A), and Mn represents the number of moles of the neutralized carboxyl groups included in the ionomer resin (A).

Illustrative examples of the metal ion which may be used for neutralizing the carboxyl group include a sodium ion, a potassium ion, a lithium ion, a zinc ion, a calcium ion, a magnesium ion, an aluminum ion and a neodymium ion. In light of the resilience performance, a magnesium ion, a zinc ion and a sodium ion are preferred.

Specific examples of the ionomer resin (A) include “Himilan® 1855” (neutralized with zinc ion), “Himilan® 1856” (neutralized with sodium ion), “Himilan® AM7327” (neutralized with zinc ion) and “Himilan® AM7331” (neutralized with sodium ion), trade names, available from Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; “Surlyn® 6320” (neutralized with magnesium ion), “Surlyn® 8120” (neutralized with sodium ion), “Surlyn® 8320” (neutralized with sodium ion), “Surlyn® 9320” (neutralized with zinc ion) and “Surlyn® 9320W” (neutralized with zinc ion), trade names, available from Du Pont Kabushiki Kaisha; and “IOTEK 7510” (neutralized with zinc ion) and “IOTEK 7520” (neutralized with zinc ion), trade names, available from EXXON Mobil Chemical Corporation.

The ionomer resin (A) and other resin may be used in combination. Illustrative examples of the polymer which may be used in combination include ionomer resins which are binary copolymers, styrene block-containing thermoplastic elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers and thermoplastic polyolefin elastomers. When used in combination, the ionomer resin is included as a principal component of the base polymer in light of the resilience performance. The percentage of the amount of the ionomer resin relative to the total amount of the base polymer is preferably no less than 50% by weight, more preferably no less than 70% by weight, and particularly preferably no less than 80% by weight.

The preferable resin which may be used in combination with the ionomer resin (A) is a styrene block-containing thermoplastic elastomer. This elastomer can be responsible for the feel at impact of the golf ball 2. This elastomer does not deteriorate the resilience performance of the golf ball 2. This elastomer has a polystyrene block as a hard segment, and a soft segment. Styrene block-containing thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenated products of SBS, hydrogenated products of SIS and hydrogenated products of SIBS. The hydrogenated products of SBS are exemplified by styrene-ethylene-butylene-styrene block copolymers (SEBS). The hydrogenated products of SIS are exemplified by styrene-ethylene-propylene-styrene block copolymers (SEPS). The hydrogenated products of SIBS are exemplified by styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In the present invention, the styrene block-containing thermoplastic elastomers include alloys of one or at least two selected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS and SEEPS, and hydrogenated products thereof with an olefin. The olefin component in this alloy is speculated to be responsible for miscibility with the ionomer resin (A). By using this alloy, the resilience performance of the golf ball 2 is improved. Preferably, an olefin having 2 to 10 carbon atoms is used. Illustrative examples of suitable olefin include ethylene, propylene, butene and pentene. Ethylene and propylene are particularly preferred.

Illustrative examples of the fatty acid component of the fatty acid metal salt (B) include stearic acid (carbon number: 18), oleic acid (carbon number: 18), linoleic acid (carbon number: 18), linolenic acid (carbon number: 18), 12-hydroxystearic acid (carbon number: 18), arachidic acid (carbon number: 20), arachidonic acid (carbon number: 20), behenic acid (carbon number: 22), lignoceric acid (carbon number: 24), nervonic acid (carbon number: 24), cerotic acid (carbon number: 26), montanic acid (carbon number: 28) and melissic acid (carbon number: 30). In light of the resilience performance of the golf ball and the flow property of the resin composition, stearic acid, behenic acid and montanic acid are preferred. Fatty acids have 18 to 30 carbon atoms. Fatty acids having no more than 28 carbon atoms are particularly preferred.

In the fatty acid metal salt (B), the fatty acid is neutralized with a metal ion. Illustrative examples of preferable metal ion include a sodium ion, a potassium ion, a lithium ion, a magnesium ion, a calcium ion, a barium ion, a zinc ion, a cadmium ion and an aluminum ion. In light of the resilience performance, a magnesium ion, a calcium ion and a barium ion are preferred.

Specific examples of the fatty acid metal salt (B) include magnesium stearate, calcium stearate, barium stearate, magnesium behenate, calcium behenate, barium behenate, magnesium montanate, calcium montanate and barium montanate. Magnesium stearate and magnesium behenate are preferred. Magnesium behenate is particularly preferred. At least two fatty acid metal salts (B) may be used in combination.

In light of the resilience performance of the golf ball 2, the amount of the fatty acid metal salt (B) is preferably no less than 25 parts by weight, more preferably no less than 33 parts by weight, and particularly preferably no less than 50 parts by weight relative to 100 parts by weight of the ionomer resin (A). In light of the durability of the golf ball 2, the amount of the fatty acid metal salt (B) is preferably no greater than 100 parts by weight, more preferably no greater than 95 parts by weight, and particularly preferably no greater than 90 parts by weight.

Preferably, the mid layer 6 contains powders of a highly dense metal. The powders serve in attaining a high specific gravity of the mid layer 6. The powders serve in attaining a great moment of inertia. The spin is suppressed according to this golf ball 2. The high resilience due to the high degree of neutralization, and the low spin due to the great moment of inertia serve in attaining a great flight distance according to this golf ball 2.

Typical highly dense metals are tungsten and molybdenum. The amount of the powders of the highly dense metal is preferably no less than 20 parts by weight, more preferably no less than 22 parts by weight, and particularly preferably no less than 24 parts by weight relative to 100 parts by weight of the base polymer of the mid layer 6. In light of ease in molding the mid layer 6, the amount of the powders is preferably no greater than 50 parts by weight. The powders have a specific gravity of preferably no less than 10, and particularly preferably no less than 15.

Into the mid layer 6 may be blended a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorbent, a light stabilizer, a fluorescent agent, a fluorescent brightening agent and the like in an appropriate amount as needed. For forming the mid layer 6, a known procedure such as injection molding, compression molding or the like may be employed.

The ratio (Pn2/Pn1) of the degree of neutralization Pn2 of the resin composition to the degree of neutralization Pn1 of the ionomer resin (A) not mixed with the fatty acid metal salt (B) is preferably no less than 1.2, and particularly preferably no less than 1.3.

In light of the resilience performance, the hardness Hm of the mid layer 6 is preferably no less than 30, more preferably no less than 40, and particularly preferably no less than 50. In light of the feel at impact, the hardness Hm is preferably no greater than 60, more preferably no greater than 58, and particularly preferably no greater than 55. The hardness Hm is measured with a Shore D type spring hardness scale attached to an automated rubber hardness tester (Kobunshi Keiki Co., Ltd., trade name “P1”) in accordance with a standard of “ASTM-D 2240-68”. For the measurement, a slab formed by hot press is used having a thickness of about 2 mm. The slab which had been stored at a temperature of 23° C. for two weeks is used for the measurement. When the measurement is carried out, three pieces of the slab are overlaid. The slab constituted with the same resin composition as that of the mid layer 6 is used for the measurement.

The specific gravity Gm of the mid layer 6 is preferably no less than 1.05. This mid layer 6 serves in attaining a great moment of inertia. In this respect, specific gravity Gm is more preferably no less than 1.10, and particularly preferably no less than 1.15. The specific gravity Gm is preferably no greater than 2.0.

In light of attainment of a great moment of inertia, the thickness Tm of the mid layer 6 is preferably no less than 0.5 mm, more preferably no less than 0.6 mm, and particularly preferably no less than 0.7 mm. In light of formability of the core 4 having a sufficient diameter, the thickness Tm is preferably no greater than 1.6 mm, more preferably no greater than 1.2 mm, and particularly preferably no greater than 1.0 mm.

In light of attainment of a great moment of inertia, the volume Vm of the mid layer 6 is preferably no less than 3.5 cm³, more preferably no less than 3.8 cm³, and particularly preferably no less than 4.1 cm³. In light of possibility of providing the core 4 having a sufficient diameter, the volume Vm is preferably no greater than 5 cm³.

In light of attainment of a great moment of inertia, the weight Wm of the mid layer 6 is preferably no less than 3.8 g, more preferably no less than 4.2 g, and particularly preferably no less than 4.5 g. In light of possibility of providing the core 4 having a sufficient diameter, the weight Wm is preferably no greater than 7 g.

The melt flow rate of the resin composition of the mid layer 6 is preferably no less than 1.0 g/10 min, more preferably no less than 3.0 g/10 min, and particularly preferably no less than 3.5 g/10 min. The melt flow rate is preferably no greater than 50 g/10 min, more preferably no greater than 40 g/10 min, and particularly preferably no greater than 30 g/10 min. The melt flow rate is measured according to “JIS K 7210” under the following conditions:

-   -   temperature: 190° C.; and     -   load: 2.16 kgf.

The modulus of flexural rigidity of the resin composition of the mid layer 6 is preferably no less than 100 MPa, more preferably no less than 110 MPa, and particularly preferably no less than 120 MPa. The modulus of flexural rigidity is preferably no greater than 300 MPa, more preferably no greater than 250 MPa, and particularly preferably no greater than 200 MPa. The modulus of flexural rigidity is measured in accordance with a standard of “JIS K 7106”.

The modulus of impact resilience of the resin composition of the mid layer 6 is preferably no less than 40%, more preferably no less than 50%, and particularly preferably no less than 55%. The modulus of impact resilience is preferably no greater than 100%, more preferably no greater than 80%, and particularly preferably no greater than 70%. The modulus of impact resilience is measured in accordance with a standard of “JIS K 6255” with the following procedure.

(1) A sheet is produced by hot press of the resin composition of the mid layer. The sheet has a thickness of 2 mm.

(2) This sheet is punched out to give a disk shape. The disk has a diameter of 28 mm.

(3) Six pieces of this disk are overlaid to produce a test piece having a cylindrical shape.

(4) This test piece is subjected to a Lubke impact resilience test.

For the cover 8, a resin composition is suitably used. Illustrative examples of the base polymer of this resin composition include ionomer resins, styrene block-containing thermoplastic elastomers, ethylene-methacrylic acid copolymers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers and thermoplastic polyolefin elastomers. In particular, an ionomer resin is preferred. The ionomer resins are highly elastic. The golf ball 2 having a cover 8 in which an ionomer resin is used in excellent in the resilience performance.

Examples of preferred ionomer resin include binary copolymers of an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Preferable binary copolymer includes a 80% by weight or more and 90% by weight or less α-olefin, and a 10% by weight or more and 20% by weight or less α,β-unsaturated carboxylic acid. This binary copolymer provides an excellent resilience performance. Examples of other ionomer resin preferred include ternary copolymers of an α-olefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and an α,β-unsaturated carboxylic acid ester having 2 to 22 carbon atoms. Preferable ternary copolymer includes a 70% by weight or more and 85% by weight or less α-olefin, a 5% by weight or more and 30% by weight or less α,β-unsaturated carboxylic acid, and a 1% by weight or more and 25% by weight or less α,β-unsaturated carboxylic acid ester. This ternary copolymer provides an excellent resilience performance. In these binary copolymer and ternary copolymer, preferable α-olefin is ethylene and propylene, and preferable α,β-unsaturated carboxylic acid is acrylic acid and methacrylic acid. Particularly preferred ionomer resin is a copolymer of ethylene, and acrylic acid or methacrylic acid.

In these binary copolymer and ternary copolymer, a part of the carboxyl groups are neutralized with a metal ion. Illustrative examples of the metal ion for use in the neutralization include a sodium ion, a potassium ion, a lithium ion, a zinc ion, a calcium ion, a magnesium ion, an aluminum ion and a neodymium ion. The neutralization may be carried out with two or more kinds of the metal ions. Particularly suitable metal ion in light of the resilience performance and durability of the golf ball 2 is a sodium ion, a zinc ion, a lithium ion and a magnesium ion.

Specific examples of the ionomer resin include “Himilan® 1555”, “Himilan® 1557”, “Himilan® 1605”, “Himilan® 1706”, “Himilan® 1707”, “Himilan® 1856”, “Himilan® 1855”, “Himilan® AM7311”, “Himilan® AM7315”, “Himilan® AM7317”, “Himilan® AM7318”, “Himilan® MK7320” and “Himilan® MK7329”, trade names, available from Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; “Surlyn® 6120”, “Surlyn® 6320”, “Surlyn® 6910”, “Surlyn® 7930”, “Surlyn® 7940”, “Surlyn® 8140”, “Surlyn® 8150”, “Surlyn® 8940”, “Surlyn® 8945”, “Surlyn® 9120”, “Surlyn® 9150”, “Surlyn® 9910”, “Surlyn® 9945”, “Surlyn® AD8546”, “HPF 1000” and “HPF 2000”, trade names, available from Du Pont Kabushiki Kaisha; and “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000” and “IOTEK 8030”, trade names, available from EXXON Mobil Chemical Corporation. Two or more kinds of the ionomer resins may be used in combination An ionomer resin neutralized with a monovalent metal ion and an ionomer resin neutralized with a bivalent metal ion may be used in combination.

The ionomer resin may be used in combination with other resin. When thus used in combination, the ionomer resin is included as a principal component of the base polymer in light of the resilience performance. The percentage of the amount of the ionomer resin relative to the total amount of the base polymer is preferably no less than 50% by weight, more preferably no less than 60% by weight, and particularly preferably no less than 70% by weight.

A preferable resin which may be used in combination with the ionomer resin is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer described above in connection with the mid layer 6 may be used in the cover 8. Specific examples of the styrene block-containing thermoplastic elastomer include “Rabalon® T3221C”, “Rabalon® T3339C”, “Rabalon® SJ4400N”, “Rabalon® SJ5400N”, “Rabalon® SJ6400N”, “Rabalon® SJ7400N”, “Rabalon® SJ8400N”, “Rabalon® SJ9400N” and “Rabalon® SR04”, trade names, available from Mitsubishi Chemical Corporation. Other specific examples of the styrene block-containing thermoplastic elastomer include “Epofriend A1010”, trade name, available from Daicel Chemical Industries; and “Septon HG-252”, trade name, available from Kuraray Co., Ltd.

When the ionomer resin and the styrene block-containing thermoplastic elastomer are used in combination in the cover 8, the percentage of the amount of the styrene block-containing thermoplastic elastomer relative to the total amount of the base polymer is preferably no less than 2% by weight, more preferably no less than 4% by weight, and particularly preferably no less than 5% by weight. This percentage is preferably no greater than 30% by weight, more preferably no greater than 25% by weight, and particularly preferably no greater than 20% by weight.

Other preferable resin which may be used in combination with the ionomer resin is an ethylene-(meth)acrylic acid copolymer. This copolymer is obtained by a copolymerization reaction of a monomer composition containing ethylene and (meth)acrylic acid. In this copolymer, a part of carboxyl groups are neutralized with a metal ion. This copolymer contains 3% by weight or more and 25% by weight or less (meth)acrylic acid component. Ethylene-(meth)acrylic acid copolymers having a polar functional group are particularly preferred.

When the ionomer resin and the ethylene-(meth)acrylic acid copolymer are used in combination in the cover 8, the percentage of the amount of the ethylene-(meth)acrylic acid copolymer relative to the total amount of the base polymer is preferably no less than 5% by weight, more preferably no less than 10% by weight, and particularly preferably no less than 15% by weight. This percentage is preferably no greater than 50% by weight, more preferably no greater than 40% by weight, and particularly preferably no greater than 35% by weight.

Specific examples of the ethylene-(meth)acrylic acid copolymer include “Nucrel®”, trade name, available from Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

Into the cover 8 may be blended a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorbent, a light stabilizer, a fluorescent agent, a fluorescent brightening agent and the like in an appropriate amount as needed. For forming the cover 8, a known procedure such as injection molding, compression molding or the like may be employed. Dimples 10 are formed by way of a large number of pimples formed on the cavity face of the mold when the cover 8 is molded.

In light of suppression of the spin, the hardness Hc of the cover 8 is preferably no less than 50, more preferably no less than 55, and particularly preferably no less than 58. In light of the feel at impact, the hardness Hc is preferably no greater than 75, more preferably no greater than 70, and particularly preferably no greater than 65. The hardness Hc is measured with a Shore D type spring hardness scale attached to an automated rubber hardness tester (Kobunshi Keiki Co., Ltd., trade name “P1”) in accordance with a standard of “ASTM-D 2240-68”. For the measurement, a slab formed by hot press is used having a thickness of about 2 mm. The slab which had been stored at a temperature of 23° C. for two weeks is used for the measurement. When the measurement is carried out, three pieces of the slab are overlaid. The slab constituted with the same resin composition as that of the cover 8 is used for the measurement.

In light of suppression of the spin, the specific gravity Gc of the cover 8 is preferably no less than 0.90, more preferably no less than 0.93, and particularly preferably no less than 0.96. In light of the formability of the cover 8, the specific gravity Gc is preferably no greater than 1.05, and particularly preferably no greater than 1.00.

In light of ease in molding the cover 8, the thickness Tc of the cover 8 is preferably no less than 0.3 mm, and particularly preferably no less than 0.4 mm. In light of outward location of the mid layer 6, the thickness Tc is preferably no greater than 2.0 mm, more preferably no greater than 1.5 mm, and particularly preferably no greater than 1.0 mm.

In light of the durability of the golf ball 2, the volume Vc of the cover 8 is preferably no less than 3.8 cm³, more preferably no less than 4.1 cm³, and particularly preferably no less than no less than 4.4 cm³. In light of possibility of providing the mid layer 6 having a sufficient thickness, the volume Vc is preferably no greater than 6.0 cm³, and particularly preferably no greater than 5.0 cm³.

In light of the durability of the golf ball 2, the weight Wc of the cover 8 is preferably no less than 3.4 g, more preferably no less than 3.8 g, and particularly preferably no less than 4.1 g. In light of possibility of providing the mid layer 6 having a sufficient thickness, the weight Wc is preferably no greater than 6.0 g, and particularly preferably no greater than 5.0 g.

In light of the feel at impact, the amount of compressive deformation of the golf ball 2 is preferably no less than 2.5 mm, more preferably no less than 2.7 mm, and particularly preferably no less than 2.9 mm. In light of the resilience performance, the amount of compressive deformation is preferably no greater than 3.8 mm, more preferably no greater than 3.5 mm, and particularly preferably no greater than 3.4 mm.

In this golf ball 2, the weight Wm of the mid layer 6 is greater than the weight Wc of the cover 8. This means that the specific gravity Gm of the mid layer 6 is sufficiently great, whereas the thickness of the cover 8 is sufficiently small. In this golf ball 2, the weight distribution is biased outwardly. According to this golf ball 2 the back spin and the side spin are suppressed. Due to a low back spin rate, a great flight distance of the golf ball 2 is attained. Due to a low side spin rate, variance of the flight direction of the golf ball 2 can be suppressed. In these respects, the difference (Wm−Wc) is preferably no less than 0.1 g, more preferably no less than 0.3 g, and particularly preferably no less than 0.4 g. The difference (Wm−Wc) is preferably no greater than 3 g.

In this golf ball 2, the volume Vm of the mid layer 6 is less than the volume Vc of the cover 8. According to this golf ball, the mid layer 6 is located outward. Due to a synergistic effect with a great specific gravity of the mid layer 6, a great moment of inertia is attained. In these respects, the difference (Vc−Vm) is preferably no less than 0.1 cm³, and particularly preferably no less than 0.2 cm³.

In this golf ball 2, the sum (Tm+Tc) of the thickness Tm of the mid layer 6 and the thickness Tc of the cover 8 is 1.0 mm or greater and 3.0 mm or less. The golf ball 2 in which the sum (Tm+Tc) is no less than 1.0 mm can be easily manufactured. In this respect, the sum (Tm+Tc) is particularly preferably no less than 1.5 mm. The golf ball 2 in which the sum (Tm+Tc) is no greater than 3.0 mm can attain a great moment of inertia. A great moment of inertia suppresses the back spin and the side spin. In this respect, the sum (Tm+Tc) is more preferably no greater than 2.5 mm, and particularly preferably no greater than 2.0 mm.

In this golf ball 2, the difference (Gm−Gc) between the specific gravity Gm of the mid layer 6 and the specific gravity Gc of the cover 8 is greater than 0. According to this golf ball 2, the spin is suppressed, and a soft feel at impact is achieved. In this respect, the difference (Gm−Gc) is more preferably no less than 0.10, and particularly preferably no less than 0.15. The difference (Gm−Gc) is preferably no greater than 1.15.

The sum total (Vm+Vc) of the volume Vm of the mid layer 6 and the volume Vc of the cover 8 is no greater than 10.0 cm³. The golf ball 2 in which the sum total (Vm+Vc) is no greater than 10.0 cm³ can attain a great moment of inertia. A great moment of inertia suppresses the back spin and the side spin. In this respect, the sum total (Vm+Vc) is preferably no greater than 9.8 cm³, and particularly preferably no greater than 9.5 cm³. The sum total (Vm+Vc) is preferably no less than 6.7 cm³.

In light of the feel at impact, the amount of compressive deformation Db of the golf ball 2 is preferably no less than 2.8 mm, more preferably no less than 3.0 mm, and particularly preferably no less than 3.2 mm. In light of the resilience performance, the amount of compressive deformation Db is preferably no greater than 3.9 mm, more preferably no greater than 3.7 mm, and particularly preferably no greater than 3.5 mm.

Upon measurement of the amount of compressive deformation Db, the golf ball 2 is placed on a hard plate made of metal. A cylinder made of metal gradually descends toward this golf ball 2. The golf ball 2 interposed between the bottom face of the cylinder and the hard plate is deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the golf ball 2 up to the state in which a final load of 1,274 N is applied thereto is measured.

EXAMPLES Example 1

A rubber composition (1) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (“BR-730”, trade name, available from JSR Corporation), 30.0 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, 12.3 parts by weight of barium sulfate, 0.9 parts by weight of bispentabromophenyl disulfide (Sankyo Kasei Co., Ltd.) and 0.7 parts by weight of dicumyl peroxide (NOF Corporation). This rubber composition (1) was placed into a mold having upper and lower mold half each having a hemispherical cavity, and compressed with heating to obtain a core. This core had a diameter of 39.6 mm.

A resin composition (A) was obtained by kneading 100 parts by weight of an ionomer resin (“Himilan® AM7327”, supra), 70 parts by weight of magnesium behenate (Nitto Kasei Kogyo K. K.) and 29 parts by weight of tungsten powders in a biaxial kneading extruder. The core was placed into a mold having upper and lower mold half each having a hemispherical cavity. The resin composition (A) was injected around the core by injection molding, whereby a mid layer was formed. This mid layer had a thickness of 0.8 mm.

A resin composition (G) was obtained by kneading 30 parts by weight of an ionomer resin (“Surlyn® 8945”, supra), 40 parts by weight of other ionomer resin (“Himilan® AM7329”, supra), 30 parts by weight of an ethylene-acrylic acid copolymer (“Nucrel® N1050H”, supra), 3 parts by weight of titanium dioxide and 0.04 parts by weight of ultramarine blue in a biaxial kneading extruder. A sphere composed of the core and the mid layer was placed into a final mold having upper and lower mold half each having a hemispherical cavity and being provided with pimples on the cavity face thereof. The resin composition (G) was injected around the sphere by injection molding, whereby a cover was formed. This cover had a thickness of 0.8 mm. A large number of dimples with a shape inverted from the shape of the pimple were formed on the cover. A clear paint including a two-component cured polyurethane as a base material was applied on this cover to give a golf ball of Example 1 having a diameter of 42.8 mm and a weight of about 45.6 g.

Examples 2 to 7 and Comparative Examples 1 to 7

Golf balls of Examples 2 to 7 and Comparative Examples to 7 were obtained in a similar manner to Example 1 except that specifications of the core, the mid layer and the cover were as listed in Tables 4 to 6 below. Specifications of the rubber compositions of the core are presented in Tables 1 and 2 below. Details of the resin compositions of the mid layer and the cover are shown in Table 3 below.

Example 8

A rubber composition (3) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, supra), 6.0 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, 5.0 parts by weight of barium sulfate, 0.9 parts by weight of bispentabromophenyl disulfide and 0.7 parts by weight of dicumyl peroxide. This rubber composition (3) was placed into a mold having upper and lower mold half each having a hemispherical cavity, and heated at a temperature of 170° C. for 15 minutes to obtain a center having a diameter of 15 mm.

A rubber composition (4) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (“BR-730”, supra), 35 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, 11.4 parts by weight of barium sulfate, 0.9 parts by weight of bispentabromophenyl disulfide and 0.7 parts by weight of dicumyl peroxide. Half shells were formed from this rubber composition (4). The aforementioned center was covered by two pieces of the half shell. The center and the half shells were placed into a mold having upper and lower mold half each having a hemispherical cavity, and heated at a temperature of 170° C. for 20 min to obtain a core having a diameter of 39.6 mm. An envelope layer was formed from the rubber composition (4).

This core was covered by the mid layer similarly to Example 1. This mid layer was covered by the cover also similarly to Example 1. A clear paint was applied to this cover similarly to Example 1 to obtain a golf ball of Example 8.

Examples 9 and 10

Golf balls of Example 9 and 10 were obtained in a similar manner to Example 8 except that specifications of the center, the envelope layer, the mid layer and the cover were as listed in Table 7 below. Specifications of the rubber compositions of the core are presented in Tables 1 and 2 below. Details of the resin compositions of the mid layer and the cover are shown in Table 3 below.

[Evaluation of Flight Distance]

A driver with a titanium head (SRI Sports Limited, trade name “XXIO”, shaft hardness: S, loft angle: 10.0°) was attached to a swing machine available from True Temper Co. Then the golf ball was hit under a condition to give the head speed of 45 m/sec, and the distance from the launching point to the point where the ball stopped was measured. Mean values of the data obtained by measuring 10 times are shown in Tables 4 to 7 below.

[Evaluation of Directional Stability]

A driver with a titanium head (SRI Sports Limited, trade name “XXIO”, shaft hardness: S, loft angle: 10.0°) was attached to a swing machine available from True Temper Co. so as to have a 2° open face. Then the golf ball was hit under a condition to give the head speed of 45 m/sec, and the distance between the landing point and a line in the intended direction was measured. The measurement was carried out 10 times to derive a first mean value. The driver was attached so as to have a 2° closed face. Then the golf ball was hit under a condition to give the head speed of 45 m/sec, and the distance between the landing point and a line in the intended direction was measured. The measurement was carried out 10 times to derive a second mean value. The sum total of the first mean value and the second mean value (i.e., deflection) is shown in Tables 4 to 7 below.

[Evaluation of Durability]A driver with a titanium head was attached to a swing machine available from True Temper Co. The golf ball was hit repeatedly in an environment of a temperature being 23° C. and under a condition to give the head speed of 45 m/sec. The number of times of the hitting until the golf ball was broken was counted. Mean values of the data obtained by measuring 12 times are shown in Tables 4 to 7 below.

TABLE 1 Composition of Core Type (1) (2) (3) (4) (5) (6) BR-730 100 100 100 100 100 100 Zinc diacrylate 30.0 30.0 6.0 35.0 37.0 40.0 Zinc oxide 5 5 5 5 5 5 Barium sulfate 12.3 15.9 5.0 11.4 11.5 13.3 Bispentabromophenyl disulfide 0.9 0.9 0.9 0.9 0.9 0.9 Dicumyl peroxide 0.7 0.7 0.7 0.7 0.7 0.7 Specific gravity 1.124 1.144 1.016 1.130 1.135 1.151

TABLE 2 Composition of Core type (7) (8) (9) (10) (11) BR-730 100 100 100 100 100 Zinc diacrylate 2.95 29.5 28.5 28.0 32.0 Zinc oxide 5 5 5 5 5 Barium sulfate 14.3 12.2 12.9 16.0 9.2 Bispentabromophenyl 0.9 0.9 0.9 0.9 0.9 disulfide Dicumyl peroxide 0.7 0.7 0.7 0.7 0.7 Specific gravity 1.134 1.122 1.124 1.140 1.118

TABLE 3 Composition of Mid Layer and Cover Type (A) (B) (C) (D) (E) (F) (G) (H) (I) Surlyn ® 8945 — — — — 48 — 30 25 30 Himilan ® AM7329 — — — — 30 — 40 35 40 Rabalon ® T3221C — — — — 22 — — 10 — Nucrel ® N1050H — — — — — — 30 30 30 Himilan ® AM7327 100 100 100 100 — 100 — — — Magnesium — — — 80 — — — — — stearate Magnesium 70 33 20 — — 70 — — — behenate Titanium dioxide — — — — — — 3 3 3 Tungsten 29 24 22 30 23 — — — 21 Ultramarine blue — — — — — — 0.04 0.04 0.04 Hardness 54 51 50 52 53 54 61 56 61 (Shore D) Specific gravity 1.15 1.15 1.15 1.15 1.15 0.99 0.97 0.97 1.15

Details of the materials shown in Table 3 are as in the following.

Surlyn® 8945:

Ionomer resin (Du Pont Kabushiki Kaisha); and

an ethylene-methacrylic acid copolymer neutralized with a sodium ion,

Himilan® AM7329:

Ionomer resin (Du Pont-MITSUI POLYCHEMICALS Co., Ltd.); and

an ethylene-methacrylic acid copolymer neutralized with a zinc ion,

Rabalon® T3221C:

Styrene block-containing thermoplastic elastomer (Mitsubishi Chemical Corporation),

Nucrel® N1050H:

Ethylene-methacrylic acid copolymer (Du Pont-MITSUI POLYCHEMICALS Co., Ltd.),

Himilan® AM7327:

Ionomer resin (Du Pont-MITSUI POLYCHEMICALS Co., Ltd.); and

an ethylene-methacrylic acid-butyl acrylate copolymer neutralized with a zinc ion,

Magnesium Stearate:

Nitto Kasei Kogyo K. K.,

Magnesium Behenate:

Nitto Kasei Kogyo K. K.

TABLE 4 Evaluation Results Compa. Compa. Example 1 Example 1 Example 2 Example 2 Example 3 Core Composition (1)   (2)   (1)   (1)   (1)   Diameter (mm) 39.6  39.6  39.6  39.6  39.6  Hardness Ho (JIS-C) 59   59   59   59   59   Hardness Hs (JIS-C) 81   81   81   81   81   Hs − Ho 22   22   22   22   22   Specific gravity  1.124  1.144  1.124  1.124  1.124 Mid Composition (A) (F) (B) (C) (D) layer Hardness Hm (Shore D) 54   54   51   50   52   External diameter (mm) 41.2  41.2  41.2  41.2  41.2  Thickness Tm (mm) 0.8 0.8 0.8 0.8 0.8 Volume Vm (cm³) 4.1 4.1 4.1 4.1 4.1 Specific gravity Gm  1.15  0.99  1.15  1.15  1.15 Weight Wm (g) 4.7 4.1 4.7 4.7 4.7 Cover Composition (G) (G) (G) (G) (G) Hardness Hc (Shore D) 61   61   61   61   61   Thickness Tc (mm) 0.8 0.8 0.8 0.8 0.8 Volume Vc (cm³) 4.4 4.4 4.4 4.4 4.4 Specific gravity Gc  0.97  0.97  0.97  0.97  0.97 Weight Wc (g) 4.3 4.3 4.3 4.3 4.3 Wm + Wc 9.0 8.4 9.0 9.0 9.0 Vm + Vc 8.5 8.5 8.5 8.5 8.5 Gm − Gc  0.18  0.02  0.18  0.18  0.18 Deformation Db (mm) 3.3 3.3 3.4 3.4 3.3 Flight distance (m) 237    234    235    232    236    Deflection (m) 3.7 8.3 3.9 4.4 3.8 Durability 100    105    105    109    97  

TABLE 5 Evaluation Results Compa. Compa. Compa. Example 3 Example 4 Example 4 Example 5 Example 5 Core Composition (1)   (1)   (7)   (8)   (9)   Diameter (mm) 39.6  39.6  38.8  38.8  39.6  Hardness Ho (JIS-C) 59   59   58   58   65   Hardness Hs (JIS-C) 81   81   80   80   77   Hs − Ho 22   22   22   22   12   Specific gravity  1.124  1.124  1.134  1.122  1.124 Mid Composition (E) (A) (A) (A) (A) layer Hardness Hm (Shore D) 53   54   54   54   54   External diameter (mm) 41.2  41.2  40.4  41.2  41.2  Thickness Tm (mm) 0.8 0.8 0.8 1.2 0.8 Volume Vm (cm³) 4.1 4.1 3.9 6.0 4.1 Specific gravity Gm  1.15  1.15  1.15  1.15  1.15 Weight Wm (g) 4.7 4.7 4.5 6.9 4.7 Cover Composition (G) (H) (G) (G) (G) Hardness Hc (Shore D) 61   56   61   61   61   Thickness Tc (mm) 0.8 0.8 1.2 0.8 0.8 Volume Vc (cm³) 4.4 4.4 6.6 4.4 4.4 Specific gravity Gc  0.97  0.97  0.97  0.97  0.97 Weight Wc (g) 4.3 4.3 6.4 4.3 4.3 Wm + Wc 9.0 9.0 10.9  11.3  9.0 Vm + Vc 8.5 8.5 10.5  10.5  8.5 Gm − Gc  0.18  0.18  0.18  0.18  0.18 Deformation Db (mm) 3.3 3.4 3.3 3.3 3.3 Flight distance (m) 232    234    232    232    234    Deflection (m) 4.0 4.6 7.2 3.6 6.5 Durability 103    108    105    104    103   

TABLE 6 Evaluation Results Compa. Compa. Example 6 Example 7 Example 6 Example 7 Core Composition (1)   (8)   (10)   (11)   Diameter (mm) 39.6  38.8  36.8  40.8  Hardness Ho 59   58   57   60   (JIS-C) Hardness Hs 81   80   79   82   (JIS-C) Hs-Ho 22   22   22   22   Specific gravity  1.124  1.122  1.140  1.118 Mid Composition (F) (F) (A) (A) layer Hardness Hm 54   54   54   54   (Shore D) External 41.2  41.2  40.0  41.8  diameter (mm) Thickness 0.8 1.2 1.6 0.5 Tm (mm) Volume Vm 4.1 6.0 7.4 2.7 (cm³) Specific gravity  0.99  0.99  1.15  1.15 Gm Weight Wm (g) 4.1 6.0 8.5 3.1 Cover Composition (I) (I) (G) (G) Hardness Hc 61   61   61   61   (Shore D) Thickness Tc 0.8 0.8 1.4 0.5 (mm) Volume Vc 4.4 4.4 7.5 2.8 (cm³) Specific gravity  1.15  1.15  0.97  0.97 Gc Weight Wc (g) 5.1 5.1 7.3 2.7 Wm + Wc 9.2 11.1  15.8  5.8 Vm + Vc 8.5 10.5  15.0  5.5 Gm − Gc −0.16 −0.16  0.18  0.18 Deformation Db (mm) 3.3 3.3 3.3 3.3 Flight distance (m) 238    237    230    232    Deflection (m) 3.5 3.8 7.9 4.6 Durability 82   84   102    90  

TABLE 7 Evaluation Results Example Example 8 Example 9 10 Center Composition (3)   (3)   (3)   Diameter (mm) 15.0  18.0  23.0  Hardness Ho (JIS-C) 38   38   38   Surface hardness (JIS-C) 48   48   48   Specific gravity  1.016  1.016  1.016 Envelope Composition (4)   (5)   (6)   layer Thickness (mm) 12.3  10.8  8.3 Specific gravity  1.130  1.135  1.151 Core Hardness Hs (JIS-C) 84   86   88   Hs-Ho 46   48   50   Mid Composition (A) (A) (A) layer Hardness Hm (Shore D) 54   54   54   External diameter (mm) 41.2  41.2  41.2  Thickness Tm (mm) 0.8 0.8 0.8 Volume Vm (cm³) 4.1 4.1 4.1 Specific gravity Gm  1.15  1.15  1.15 Weight Wm (g) 4.7 4.7 4.7 Cover Composition (G) (G) (G) Hardness Hc (Shore D) 61   61   61   Thickness Tc (mm) 0.8 0.8 0.8 Volume Vc (cm³) 4.4 4.4 4.4 Specific gravity Gc  0.97  0.97  0.97 Weight Wc (g) 4.3 4.3 4.3 Wm + Wc 9.0 9.0 9.0 Vm + Vc 8.5 8.5 8.5 Gm − Gc  0.18  0.18  0.18 Deformation Db (mm) 3.3 3.4 3.4 Flight distance (m) 240    238    237    Deflection (m) 3.4 3.2 3.2 Durability 98   95   93   As is shown in Tables 4 to 7, the golf ball of each Example is excellent in various performances. Therefore, advantages of the present invention are clearly suggested by these results of evaluation.

The golf ball according to the present invention can be used for the play at the golf course, and the practice at the driving range. The foregoing description is just for illustrative examples; therefore, various modifications can be made in the scope without departing from the principles of the present invention. 

1. A golf ball comprising a core, a mid layer situated on the external side of the core, and a cover situated on the external side of the mid layer, the weight Wm of the mid layer being greater than the weight Wc of the cover, the sum (Tm+Tc) of the thickness Tm of the mid layer and the thickness Tc of the cover being 1.0 mm or greater and 3.0 mm or less, the mid layer being constituted with a resin composition, the resin composition comprising 100 parts by weight of an ionomer resin (A) and 25 parts by weight or more and 100 parts by weight or less of a fatty acid metal salt (B), the ionomer resin (A) being a neutralized product of a ternary copolymer of ethylene, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and an α,β-unsaturated carboxylic acid ester, with a metal ion, and the fatty acid metal salt (B) having 18 to 30 carbon atoms.
 2. The golf ball according to claim 1, wherein the difference (Wm−Wc) between the weight Wm and the weight Wc is 0.1 g or greater and 3 g or less.
 3. The golf ball according to claim 1, wherein the difference (Gm−Gc) between the specific gravity Gm of the mid layer and the specific gravity Gc of the cover is greater than 0 and no greater than 1.15.
 4. The golf ball according to claim 1, wherein the specific gravity Gm of the mid layer is 1.05 or greater and 2.0 or less.
 5. The golf ball according to claim 1, wherein the specific gravity Gc of the cover is 0.90 or greater and 1.05 or less.
 6. The golf ball according to claim 1, wherein the volume Vm of the mid layer is less than the volume Vc of the cover.
 7. The golf ball according to claim 6, wherein the difference (Vc−Vm) between the volume Vc and the volume Vm is no less than 0.1 cm³.
 8. The golf ball according to claim 1, wherein: the metal ion of the ionomer resin (A) is a magnesium ion, a zinc ion or a sodium ion; the fatty acid component of the fatty acid metal salt (B) is stearic acid, behenic acid or montanic acid; and the metal ion of the fatty acid metal salt (B) is a magnesium ion, a calcium ion or a barium ion.
 9. The golf ball according to claim 1, wherein the Shore D hardness Hm of the mid layer is 30 or greater and 60 or less.
 10. The golf ball according to claim 1, wherein the resin composition of the mid layer has a melt flow rate of 1.0 g/10 min or greater and 50 g/10 min or less.
 11. The golf ball according to claim 1, wherein the resin composition of the mid layer has a modulus of flexural rigidity of 100 MPa or greater and 300 MPa or less.
 12. The golf ball according to claim 1, wherein the resin composition of the mid layer has a modulus of impact resilience of no less than 40%.
 13. The golf ball according to claim 1, wherein the ratio (Pn2/Pn1) of the degree of neutralization Pn2 of the resin composition of the mid layer to the degree of neutralization Pn1 of the ionomer resin (A) not mixed with the fatty acid metal salt (B) is no less than 1.2.
 14. The golf ball according to claim 1, wherein the difference (Hs−Ho) between the JIS-C hardness Hs of the surface of the core and the JIS-C hardness Ho of the central point of the core is no less than
 20. 15. The golf ball according to claim 1, wherein the JIS-C hardness Ho of the central point of the core is 25 or greater and 70 or less.
 16. The golf ball according to claim 1, wherein the JIS-C hardness Hs of the surface of the core is 60 or greater and 95 or less.
 17. The golf ball according to claim 1, wherein the core has a specific gravity of 1.00 or greater and 1.20 or less.
 18. The golf ball according to claim 1, wherein the thickness Tc of the cover is no greater than 2.0 mm.
 19. The golf ball according to claim 1, wherein the Shore D hardness of the cover is 50 or greater and 75 or less.
 20. The golf ball according to claim 1, wherein the core has a center, and an envelope layer situated on the external side of the center. 